In the early 1970's Motor Wheel Corporation of Lansing, Michigan, assignee of applicant herein, as well as its then parent company, The Goodyear Tire & Rubber Company of Akron, Ohio, developed and introduced an improved form of composite metal-elastomer styled wheel marketed under the trademark "POLYCAST" of Motor Wheel Corporation (assignee of record herein). Such styled wheels provided an automotive type wheel in which metallic parts of simple, conventional configuration, for which manufacturing equipment was already available, are utilized as the basic structural components to thereby obtain a high strength standardized wheel construction at minimum cost. The aesthetically pleasing appearance was imparted to this standard steel backbone by a permanently adhered ornamental plastic body, either molded separately or in-situ as a homogeneous one-piece body, or in the form of a plastic cover secured by an adhesive foam body to the outboard side of the wheel. This provided an improved anti-noise characteristic to the wheel and enabled the appearance of the wheel to be readily and economically varied to suit different customer's styling requirements without varying the basic structural components of the wheel. Additional benefits resided in the side impact cushioning to prevent damage to the wheel while retaining the high strength and impact resistance advantages of the time-proven conventional ductile steel wheel components. Wider variations in styling and contour configurations were also obtained than were possible in the previous deep drawn styled all-steel wheels. The serious problems of loss and theft of removable wheel covers were also eliminated.
Various embodiments of such "POLYCAST" wheels, as well as methods and apparatus for producing the same, are set forth in the following United States Patents assigned to the assignee herein, which are incorporated herein by reference:
U.S. Pat. No. 3,669,501, 6/1972, Derleth, PA1 U.S. Pat. No. 3,756,658, 9/1973, Adams, PA1 U.S. Pat. No. 3,762,677, 10/1973, Adams, PA1 U.S. Pat. No. 3,794,529, 2/1974, Thompson, PA1 U.S. Pat. No. 3,815,200, 6/1974, Adams, PA1 U.S. Pat. No. 3,918,762, 11/1975, Hampshire, PA1 U.S. Pat. No. 3,935,291, 1/1976, Jackson, PA1 U.S. Pat. No. 3,956,451, 5/1976, Adams, PA1 U.S. Pat. No. 4,398,770, 8/1983, Smith, PA1 U.S. Pat. No. 4,659,148, 4/1987, Grill, PA1 U.S. Pat. No. 4,682,820, 7/1987, Stalter, PA1 U.S. Pat. No. 4,786,027, 11/1988, Stalter, PA1 U.S. Pat. No. 4,790,605, 12/1988, Stalter, PA1 U.S. Pat. No. 4,847,030, 7/1989, Stalter, PA1 U.S. Pat. No. 4,861,538, 8/1989, Stalter, PA1 U.S. Pat. No. 4,963,083, 10/1990, Stalter PA1 U.S. Pat. No. 4,976,497, 12/1990, Post et al, PA1 U.S. Pat. No. 5,059,106, 10/1992, Joseph, PA1 U.S. Pat. No. 5,088,798, 2/1992, Stalter. PA1 U.S. Pat. No. 5,128,085, 7/1992, Post et al.
Other prior art patents issued to unrelated parties and directed to various types of such styled metal and plastic wheels include U.S. Pat. Nos. 3,823,982; 3,827,756; 3,894,775; 3,915,502; and 3,998,494 as well as British Patent No. 1,290,946 (1972) and German Offenlegunschrift No. 2,457,907 (June/1976).
In one embodiment of the manufacture of such POLYCAST wheels, a portion of the mold comprises a conventional metal vehicle wheel having a drop center rim secured to a central disc or body having the usual bolt circle holes and a central aperture so that the disc can be mounted on an axle, drum or disc brake assembly. The metal wheel is employed in conjunction with an upper back-up clamp and lower mold part to define therewith a sealed cavity for molding and attaching a three-dimensional contoured plastic overlay, the overlay thus being molded in-situ and permanently attached to the outboard side of the wheel in the mold apparatus. Preferably, the wheel forms the upper surface of the mold cavity and a reaction mixture of a urethane elastomer liquid adhesive material is injected or poured into the mold to fill the cavity and contact the outboard surface of the wheel assembly. The urethane material components react and solidify to form a high density non-cellular elastomer body which permanently adheres to the outboard surface of the wheel subassembly. The plastic overlay may also be molded from a lower density microcellular closed cell urethane elastomer adhesive material, or a separate decorative cover may be pre-formed and permanent adhesively adhered to the steel wheel subassembly. If a urethane material molded-in-situ embodiment is employed, which is presently preferred, the urethane is allowed to react, expand (if cellular), cure and thereby solidify in the mold cavity, and then the mold is opened so that the wheel with the overlay securely adhered to it may be removed from the mold. The overlay may then be painted or otherwise covered with a decorative coating to provide a finished metallic-appearing ornamental wheel. The urethane elastomer thus forms a plastic body having a three-dimensional contour which is permanently attached to the outboard side of the wheel to provide a decorative surface, and the elastomer overlay appears to be an integral portion of the metal wheel.
Due to the preferred orientation of the overlay beneath the wheel disc, the outboard face of the overlay is adjacent the lowermost surface of the mold cavity. Although the majority of gas bubbles evolved from the reacting components are trapped within the urethane mixture as the same is curing and gelling toward solidification, such gas bubbles tend to rise and hence some can migrate toward the inboard portion of the overlay closest to the wheel disc 24 while the reaction mixture is still liquid. This migration results in a variation in density axially of the overlay so that a relatively dense but thin skin (usually less than 0.05" in thickness) is produced adjacent the outboard face of the overlay, the urethane becoming less dense and more cellular axially towards the outboard disc surface. Hence, any molding defects, such as large pockets or bubbles, will tend to occur adjacent or at the interface of the overlay and the disc where, generally speaking, such defects are less critical and do not cause as much spoilage in the end product, as compared to casting with the mold inverted so that the mold part is superimposed on the rim and disc subassembly, which tends to locate such casting defects at the outboard face of the overlay.
Although it is generally advantageous to concentrate gas bubble defects at the wheel inboard side of the plastic overlay molded onto the metal wheel backbone, such gas-bubble-induced surface defects still can and do result in appearance defects significant enough to require scrapping and/or rework of such finished wheels after the molding operation. For example, this problem can occur in the formation by molding of the wheel vent windows and associated "scoop" protrusions in the outboard face of urethane body of the "POLYCAST" wheel. These decorative vent scoop communicate with the vent openings or windows in the steel disc of the wheel backbone in order to provide air flow ventilation through the wheel for brake cooling.
The practice typically employed in making such vent windows in the urethane plastic body has been to have the mold vent core fingers or bosses (such as bosses 46 and/or pedestal portions 294 of the aforementioned U.S. Pat. No. 3,762,677, or core bosses 200-212 of the aforementioned U.S. Pat. No. 5,059,106; or the cooperating offset cooperative pedestal cores 330 and clamp cores 332 of the aforementioned U.S. Pat. No. 4,963,083) protrude upwardly from a lower mold part and/or downwardly from the upper clamp part so as to penetrate the "windows" in the steel backbone or disc of the metal portion of the wheel to provide coring for molding of a brake ventilation air passage which extends completely through the wheel. A relatively large clearance is provided between each such mold core and encircling margin of the associated steel disc vent opening so that urethane material flows upwardly therebetween to cover the vent opening edge in the steel disc. This urethane window margin material is prevented from escaping into and onto the inboard face of the steel disc by a suitable imperforate seal construction, typically a silicone seal carried by the upper clamp part of the mold apparatus, such as the seal 236 in the aforementioned 4,963,083 patent or the lid seal 90 of the aforementioned U.S. Pat. No. 5,059,106.
Hitherto it has not been found possible to economically vent the inboard face of the plastic marginal material internally surrounding the metal marginal wall of the disc vent opening at the inboard side of the disc. Hence this as-molded inboard-facing urethane surface is often characterized by pimples, blemishes and irregular open pockets and other such visible surface irregularities resulting from the gas bubble migration upwardly against the inboard window lid seal and the concentration thereof at this window margin surface as the urethane cures and solidifies. Generally, as indicated above, these surface irregularities, although visible when viewing the inboard side of the wheel, have not been considered to be a wheel-scrapping defect inasmuch as the inboard side of the wheel is hidden from view when mounted to a vehicle. Nevertheless such gas bubbles can and do on occasion concentrate close to the inner peripheral edge of the cast urethane window-margin adjacent the lid seal. During the demolding operation, when the seal is raised from the steel disc backbone, some of such gas bubble defects can cause the inner peripheral edge of the cast urethane to break away and thereby create a jagged inner edge in the urethane window scope protrusion. Such a defect indeed is then visible from the exterior side of the wheel when viewing into the urethane air vent scope opening. Such an appearance defect when viewable from the outboard side of the wheel requires rejection of the wheel from a quality control standpoint, thereby requiring either rework of the wheel to repair the defect, if possible, or scrapping of the entire composite wheel.
Other production problems associated with molding of the urethane around the inner margin of the metal disc window include cleaning and maintenance of the window lid seal attached to or carried by the upper mold clamp part, with attendant material and labor cost, as well as the cost of replacement of such seals due to the wear of the seal after repeated cleanings.